You can run a clean up with the SVN tool (command line or via GUI). This StackOverflow Q&A describes pretty well what your options are:
Note that it depends on your SVN version what it will clean. Using a newer SVN version will have an impact on the Mendix projects you have (see the Mendix docs for the supported SVN tortoise versions).
With changes over time the .svn folder will increase in size again.
You can use the cleanup command of TortoiseSVN.
Make sure you have the option Vacuum Pristine Copies selected.